1. Field of the Invention
The present invention generally relates to a semiconductor device and a method of producing the semiconductor device. More particularly, this invention relates to a semiconductor device and a method of producing the semiconductor device that comprises a MOS transistor including a gate electrode composed of a conductive material and a side wall formed on the sides of the gate electrode; a trimming fuse composed of a conductive material; a resistance element; and an insulating film covering the MOS transistor, the trimming fuse, and the resistance element, the insulating film having a trimming opening for laser trimming in an area above the trimming fuse where the insulating film is thinner than in other areas.
2. Description of the Related Art
When designing a semiconductor device, its specifications such as electrical resistance are defined so that the device works normally under certain electrical conditions. In a step near the end of a production process of such a semiconductor device, a characteristic test is performed to determine whether the semiconductor device meets the specifications.
Generally speaking, as the integration density of a semiconductor device increases, the number of defective products in a production process increases and the production yield decreases. If such densely-integrated semiconductor devices are discarded because of minor defects, it results in a huge loss. To reduce such a loss, trimming fuses for correcting electrical characteristics are employed in some semiconductor devices.
Generally, a polysilicon film or a metal film is used as the material of a trimming fuse and a laser beam is used to cut a trimming fuse. A trimming fuse is covered by an insulating film and, normally, a portion of the insulating film is etched to form a trimming opening through which the trimming fuse is irradiated with a laser beam. The trimming opening reduces absorption of energy of the laser beam by the insulating film.
In a laser trimming process of cutting a trimming fuse by a laser beam, an underlying film may be damaged if the intensity of the laser beam is too high. To prevent damage to an underlying film, it is important to optimize the intensity of a laser beam and the thickness (hereafter called a “remaining thickness”) of the insulating film at the bottom of the trimming opening above a trimming fuse.
Meanwhile, if the insulating film in the trimming opening is entirely removed and a trimming fuse is exposed, it becomes easier to optimize the intensity of a laser beam (see, for example, patent document 1).
However, this configuration may allow moisture to penetrate into the semiconductor device through the trimming opening and may adversely affect the characteristics and reliability of a semiconductor device.
On the other hand, if the remaining thickness is large, it is necessary to increase the intensity of a laser beam to cut the trimming fuse. Such a high-intensity laser beam may scatter and adversely affect elements near the trimming opening.
Therefore, it is important to optimize the remaining thickness of the insulating film at the bottom of the trimming opening.
Meanwhile, when a large number of chips are formed on a wafer, the amount of time necessary to adjust the electrical characteristics of the chips using trimming fuses becomes an important factor determining production efficiency. If the remaining thickness varies from chip to chip, the intensity and irradiation time of a laser beam are determined based on the largest remaining thickness on the wafer. This may reduce the efficiency of a laser trimming process by a laser trimming apparatus. Therefore, it is also important to make the remaining thicknesses as uniform as possible.
Some methods have been proposed to control the remaining thickness of an insulating film at the bottom of a trimming opening.
For example, in a proposed method, a silicon dioxide film is formed as an insulating film on a trimming fuse, a silicon nitride film is formed on the insulating film as an etching stop layer, and another silicon dioxide film is formed on the silicon nitride film. The proposed method controls the thickness of the insulating film using the difference in etching rates of the silicon nitride film and the silicon dioxide film (see, for example, patent documents 2 and 3).
However, since there is not much difference in etching rates between a silicon nitride film and a silicon dioxide film, the remaining thickness of the insulating film becomes inconsistent in a multilayer wiring board and it becomes difficult to stably perform laser trimming. Also, in the proposed method, a silicon nitride film has to be formed additionally as an etching stop layer. Therefore, the proposed method increases the number of steps necessary to produce a semiconductor device.
According to another proposed method, a portion of an undermost wiring pattern (aluminum wiring pattern) of a multilayer wiring structure is used as an etching stop layer to control the remaining thickness of an insulating film at the bottom of a trimming opening above a trimming fuse (see, for example, patent document 4).
With this method, however, the insulating film under the etching stop layer may also be etched when the etching stop layer is removed and, as a result, the remaining thickness of the insulating film above the trimming fuse may become inconsistent.
Another proposed method uses a frame-shaped metal film as an etching stop layer. The frame-shaped metal film is not present in a laser-irradiation area above a trimming fuse but a part of which is exposed in a trimming opening (see, for example, patent document 5).
One problem with this method is that when the frame-shaped metal film is formed by etching a portion of a metal film in the laser-irradiation area, an insulating film in the laser-irradiation area is also etched and, as a result, the remaining thickness of the insulating film above the trimming fuse becomes inconsistent. Another problem with this method is that although the frame-shaped metal film functions as an etching stop layer during the formation of a trimming opening, no etching stop layer is provided in the laser-irradiation area. Accordingly, the insulating film in the laser-irradiation area above the trimming fuse is etched during the formation of the trimming opening and the remaining thickness of the insulating film becomes inconsistent.
In still another proposed method, a silicon dioxide film is formed above a trimming fuse and a silicon-rich-oxide (SRO) film is formed as an etching stop layer on the silicon dioxide film (see, for example, patent document 6).
With this method, however, since there is not much difference in etching rates between an SRO film and a silicon dioxide film, the remaining thickness of the silicon dioxide film becomes inconsistent in a multilayer wiring board and it becomes difficult to stably perform laser trimming. Also, in the proposed method, an SRO film has to be formed additionally as an etching stop layer. Therefore, the proposed method increases the number of steps necessary to produce a semiconductor device.
[Patent document 1] Japanese Examined Patent Application Publication No. 60-44829
[Patent document 2] Japanese Patent Application Publication No. 2001-176976
[Patent document 3] Japanese Patent Application Publication No. 2004-111420
[Patent document 4] Japanese Patent Application Publication No. 2003-258103
[Patent document 5] Japanese Patent Application Publication No. 2003-264230
[Patent document 6] Japanese Patent Application Publication No. 2005-197602